The present invention relates generally to the field of navigation systems. More specifically, the present invention relates to navigation systems for determining a navigation solution for a mobile platform such as an aircraft. More specifically still, the present invention relates to a navigation system having integrated functionality of both civil and military navigation systems.
Many military aircraft have a Global Positioning System (GPS)-based navigation system and/or other navigation system having unique anti-jamming, improved accuracy, and other capabilities. Military aircraft use the encrypted, military code (P/Y code) transmitted by GPS satellites to generate a navigation solution. A great deal of development effort has been invested in these military navigational systems over the years to integrate military code GPS receivers with other navigation devices, such as, inertial navigation units, air data, and tactical navigation devices, to form a combined, blended navigation system.
The United States Government has recently mandated that all military aircraft operating in civil airspace must meet many requirements of civil aircraft operating in the same airspace. Civil navigation systems have a GPS-based navigation system which utilize the civil code (C/A, or coarse/acquisition code) to determine a navigation solution. However, civil navigation systems are configured to perform a number of additional functions which are not required of military navigation systems, such as integrity monitoring. Typically, the Federal Aviation Administration (FAA) issues Technical Standard Orders (TSOs) requiring new functional requirements for aircraft operating in civil airspace. For example, Receiver Autonomous Integrity Monitoring (RAIM) was introduced as a requirement for all civil aircraft by the FAA in TSO-C129 in the early 1990s. Thus, existing military navigation systems must be reconfigured to perform a number of additional functions currently implemented in civil navigation systems.
As mentioned, the military code transmitted by GPS satellites is encrypted and, therefore, not available for non-U.S. military applications. The military code is also much more accurate (e.g., currently up to eight times more accurate in some systems) than the commercially available civil code. Many foreign countries have now forbidden any aircraft from utilizing the military code in their sovereign airspace since the foreign countries are unable to access the code and monitor it. This has resulted in some military aircraft having to take long detours around the sovereign airspace of these countries to reach the destination of the aircraft.
One solution to the above problems is to redesign the vast system development already done on the military navigation system to implement each of the functions required by the FAA and foreign governments. The navigation systems of the military aircraft would have to be redesigned to perform the necessary functions, utilizing the less accurate civil code as required. However, this approach is costly and inflexible. As air traffic becomes more congested, aircraft separation distances are decreasing, and the FAA is issuing additional requirements for aircraft navigation systems. Each new FAA requirement would require a new redesign of the military navigation system.
Even assuming the costly development were undertaken, additional problems persist. Although military GPS receivers are capable of using the civil code, it would be difficult to design a way of using the military code for military functions and the civil code for the required civil functions, primarily because GPS receivers cannot track both codes simultaneously. Further, a system would have to be developed for the FAA to approve and validate the military navigation system without breaching the security of proprietary military information.
Another solution is for military aircraft to avoid flying in civil airspace. While this may be possible for some aircraft, such as carrier-based fighter planes, cargo planes and surveillance planes often require clearance over civil and foreign airspace for daily, routine missions. Further, while oceanic airspace is the only place that currently authorizes GPS-only navigation, it is likely that the requirement of GPS navigation will spread to other regions of airspace in the years to come.
Accordingly, there is a need for an improved navigation system that integrates the functionality of multiple navigation systems. The integrated navigation system would be easily upgraded and relatively inexpensive to develop and implement. The integrated navigation system would also maintain much of the functionality of each component navigation system.
According to an exemplary embodiment, an integrated navigation system includes a first navigation system, a second navigation system, and an integrity check system. The first navigation system receives first positioning signals and generates a first navigation solution based on the first positioning signals. The second navigation system receives second positioning signals and generates a second navigation solution based on the second positioning signals. The integrity check system receives the first navigation solution and the second navigation solution, compares the first navigation solution to the second navigation solution, and generates a validity signal based on the comparison.
According to another exemplary embodiment, a system for providing integrity checking to an existing navigation system configured to generate a navigation solution is disclosed. The system includes a civil code GPS receiver operable in parallel with the existing navigation system. The civil code GPS receiver is configured to receive satellite signals and to generate location signals based on the satellite signals. The system further includes a means for checking the integrity of the navigation solution and for generating a validity signal representative of the integrity of the navigation solution.
According to yet another exemplary embodiment, in a navigation system having a military code GPS receiver configured to receive a military satellite signal and to generate first location signals based on the military satellite signals, an improvement for configuring the navigation system for operation in civil airspace is disclosed. The improvement includes a civil code GPS receiver configured to receive satellite signals having a civil code and to generate second location signals based on the civil code. The improvement further includes a means for receiving the second location signals and using the second location signals to perform a function required of a civil aircraft.
According to still another exemplary embodiment, a method of checking the integrity of a navigation system generating a first navigation solution based on first positioning data includes receiving second positioning data different from the first positioning data; generating a second navigation solution based on the second positioning data; comparing the second navigation solution to the first navigation solution; and generating an invalidity signal based on the comparison.
According to yet still another exemplary embodiment, an integrated navigation system includes a first receiver and a second receiver. The first receiver is configured to receive GPS satellite signals and to generate a first navigation solution. The second receiver is configured to receive the GPS satellite signals. The second receiver has a fault detection and exclusion algorithm configured to identify a faulty GPS satellite signal and to provide a fault signal to the first receiver. The first receiver is configured to exclude the faulty GPS satellite signal from the navigation solution.